The present invention relates papermaking. In particular the invention concerns a method for producing a paper web. According to a method of this kind, a fibrous raw material is slushed to form a stock, a web is formed from the stock and the web is dried.
With the aid of the present invention it is possible to produce a base paper which is particularly well suited to the manufacture of fine paper. The surface weight of a base paper of this kind is generally 20 to 200 g/m2.
High-quality printing matters, such as brochures, advertising materials and catalogues, are made from fine papers which have good opacity, an even surface structure and high brightness.
Traditionally, fine papers have been manufactured from cellulosic hardwood or softwood pulps or mixtures thereof. The problem associated with the known art is that it is not possible at low grammage to reach sufficiently high opacity for the pulp or the paper manufactured therefrom. The formation of the chemical pulp and the paper made from it remains rather poor when high opacity is aimed at.
Light, coated paper qualities containing mechanical pulp are also known in the art. These are manufacture from a mechanical pulp made from spruce and they usually contain about ⅓ to xc2xc softwood pulp which reinforces the pulp and improves the strength properties of the paper.
The particular advantages of mechanical pulps in comparison to chemical pulps are their lower production costs and greater yield. The coarse, stiff fibers of the pulp lead, however, to fiber coarsing, which shows in offset printing Further, the disadvantages of groundwood of spruce include its poor dewatering at low drainability and the large energy consumption of the whole pulping process. A problem of known papers based on groundwood is also their low brightness and poor brightness stability. They are not either shelf stable if spruce GW has been used.
Paper qualities containing mechanical pulps and combinations of mechanical pulps and chemical pulps, respectively, have not been used for fine papers. Instead, said types of papers (e.g. LWC) are primarily used as magazine papers.
It is an object of the present invention to eliminate the problems of the prior art and provide a solution for producing a base paper suitable for the manufacture of fine papers. It is also an object of the present invention to provide a fine paper of a novel kind, having the high brightness and good smoothness of traditional fine papers and which further exhibits the good opacity and excellent printability due to the good formation characteristic for mechanical printing papers.
The present invention is based on the concept of combining groundwood of hardwood and chemical pulp of softwood and of producing a base paper from a mixture of the mechanical and the chemical pulp. In connection with the present invention it has been found that a mechanical pulp (in particular Pressure Ground Wood, PGW) manufactured from aspen and other wood species of the Populus family contain a great amount of short fibers which improve the traditionally insufficient bulk and light scattering of fine paper. Although the strength properties of aspen GW, as regards, for example ScottBond strength, are not entirely sufficient, by combining aspen GW with a chemical pulp produced from softwood, it becomes possible to produce a basepaper which exhibits excellent opacity at high brightness and an even surface and good strength. Due to the good bonding strength of softwood, aspen GW can be used in an amount of up to 30 to 70% of the dry weight of the pulp.
More specifically, the solution according to the present invention is mainly characterized in that the stock is formed from a mechanical pulp of a wood raw material of the Populus family and a bleached chemical softwood pulp, the amount of mechanical pulp being 20 to 70wt-% and the amount of bleached softwood pulp being 80 to 30 weight-% of the dry matter of the stock.
Considerable advantages are obtained by means of the invention Thus, using the base paper according to the invention it is possible to obtain better opacity on the same level of brightness as that exhibited for traditional fine papers. The paper contains more fines and its bulk and opacity are greater, which gives it good printing properties, Surprisingly, we have found that by means of the present invention it has become possible at lower grammage to produce a fine paper having an extremely high brightness. When the fine papers produced from the present base paper are compared with traditional fine papers, thanks to the low grammage, a yield gain of up to about 20% can be obtained; on the same opacity level the present paper will give more printing surface per weight unit than traditional fine papers.
The short-fibered aspen gives the paper good light scattering properties.
According to the invention it is therefore possible to provide a paper which fulfills all the quality requirements of coated fine papers consisting solely of chemical pulp but which, at the same time, provides high opacity and bulk and excellent printability properties.
In the following, the invention, its features and benefits will be examined in greater detail with reference to a detailed description and a number of working examples.
The fiber structure of aspen and wood species belonging to the same family differ from the fiber structures of the hardwood species most frequently used for pulp making, such as birch. The dimensions of the aspen fiber, the fiber length and width are smaller than for spruce and birch The tracheids of aspen are smaller (length 0.9 mm) than the tracheids of birch (1.0-1.1 mm). In both, the proportion of vasculum cells is about 25%. Traditionally, the tubular cells contained in aspen have been considered to cause runability problems on the paper machine and they have not been believed to provide for bonding. As a result of the short fibers and the poor bonding of the vasculum cells, dusting of the paper can occur on the paper machine and during posttreatment.
According to the present invention it has now surprisingly been found that by using a combination of mechanical pulp produced from aspen and chemical softwood pulp, the runability problems caused by the tubular cells can be avoided and a pulp can be produced which has impeccable strength properties. Since the aspen pulp has shorter fibers than the birch pulp and even much shorter than spruce, at a given grammage there are more aspen fibers than birch or spruce fibers. This leads to a greater light scattering coefficient and bulk in the present invention. Further, the advantageous fiber length distribution gives the paper an excellent formation i.e. variation of the grammage of paper on a small scale, typically  less than 3 g/m2. The smoothness of the paper is also good.
Due to all these factors, it is now possible to achieve a base paper which can be coated for production of high-quality line papers, which have excellent printability properties
The advantages of the special aspen pulp in comparison to spruce groundwood comprise high brightness and brightness stability. The stability of the brightness is in particular due to the low lignin-content of aspen groundwood or corresponding mechanical pulp and to the low concentration of carbonyl groups compared to spruce groundwood. Further, it should be pointed out that a paper web produced from aspen has clearly better dewatering properties than a web produced from spruce. The shorter dewatering time and the higher dry matter content together give a sheet with more porosity.
The greatest advantage of the fiber distribution of aspen is obtained when the pulp has been beaten to the drainability of fine papers. It should be mentioned that spruce has to be refined to a higher degree of beating because of the stiff fibers contained therein. The long and stiff fibers of mechanical pulps produced from spruce cause a coarsing of the paper surface fibers during coating and, in particular, during printing. The phenomenon is rather typical for mechanical printing papers and it substantially deteriorates the quality of the printing surface. The requirements for the printing surface of fine papers are very strict and no coarsing of the paper surface is acceptable. With the particular aspen pulp according to the present invention, not fiber coarsing problems occur, which relates to the fact that practically all long and stiff fibers of the +14 and +28 fractions have been eliminated.
As an example of the fiber length distribution of aspen, the following table can be presented which indicates the fiber fractions retained by various sieves (mesh). The determination have been made from dosing pulps and the table compares aspen fibers to birch and spruce fibers, respectively:
The average fiber length of aspen of PGW is smaller than of spruce (FS is typically about 0.54xc2x10.01).
Preferably the mechanical aspen pulp contains about 10 to 20% of +20 . . . +48 mesh fibers, which confer mechanical strength to the pulp. In order to maximize light scattering, the portion of +100, +200 and xe2x88x92200 fractions should be as large as possible. Preferably they stand for distinctly more than 50% of the whole pulp. In particular their proportion of the whole pulp is over 70%, preferably over 80%. On the other hand, the amount of the smallest fraction, i.e. the 200 mesh, should not be too large, because then dewatering on the paper machine would become more difficult. Preferably the proportion of this fraction is smaller than 50%, in particular 45% or less.
The proportion of +14 and +28 mesh fiber fractions are below 10%, preferably below 5%, and in particular below 3%. The amounts of Pulmac shives at 0.8 mm/mg/g are below 1, in particular below 0.5.
In addition to the aspen mentioned above, pulp produced from any mechanical pulp made of a tree of the Populus family can be used for the base paper. Suitable species are, for example, P. tremula, P tremuloides, P balsanea, P. balsamifera, P. trichocarpa and P. heterophylla. A preferred embodiment comprises using aspen (trembling aspen, P. tremuta; Canadian aspen, P. tremuloides), or aspen varieties known as hybride aspens produced from different base aspens by hybridizing as well as other species produced by recombinant technology, or poplar. The raw material is processed to groundwood (GW) or pressure groundwood (PGW) or it is disintegrated to chips and the chips are used for producing thermomechanical pulp (TMP) of chemimecbanical pulp (CTMP) in a manner known per se.
The mechanical pulp is bleached after grinding or refining, respectively. Preferably the pulp is peroxide bleached at alkaline conditions. According to a preferred embodiment the pulp is bleached with a one, two or multistage bleaching sequence, the pulp being acidified between the bleaching stages and the peroxide residue being reduced. Generally the peroxide dosage is about 2 to 3.5 weight-% of the dry matter of the pulp, for aspen pulp 0.5 to 1.5%, in particular 0.7 to 1.2%. A dithionite bleaching step comprising the treatment of the pulp with Na2S2O4 can be incorporated into the peroxide bleaching sequence.
The mechanical pulp is washed before bleaching and after the bleaching with a mixture of water from the pulping section and a clarified recirculation water of the paper machine in a washing press (fabric press) by using typically about 0.1 to 10 m3 water per ton of pulp. By using the washing press water is removed from the pulp in order to increase the dry matter content of the pulp to about 20 to 30%. The waters from the dewatering are recycled to the production of the mechanical pulp. By the washing press it is possible to prevent impurities from being transferred to the paper machine.
The bleached pulp is then refined to the desired degree of beating, which is, e.g. 30 to 100 CSF, preferably about 40 to 80 CSF.
A stock is formed from the mechanical pulp together with a chemical pulp. The stock can contain other fiber materials and additives, such as fillers. Calcium carbonate is an example of a filler. The dry matter content of the stock is about 0.1 to 5% Clarified filtrate of a circulating water of the paper machine is used as the aqueous phase of the stock The chemical pulp used comprises in particular a fully bleached chemical softwood pulp, whereby a paper web suitable as a base paper of fine papers is obtained. Said web has high bulk, high brightness and high opacity and good formation. The amount of the mechanical pulp is then for example 20 to 70 weight-%, preferably 30 to 60 weight-%, and the amount of the bleached softwood pulp is for example 80 to 30 weight-%, preferably 70 to 40 weight-% of the dry matter of the stock.
Preferably the chemical pulp used for the preparation of the base paper is produced by method known as a modified batch-type cooking (Superbatch Cook). This cook is discribed in literature [cf., for example, Malinen, R. Paperi ja Puu (Paper and Timber), 75 (1993) 14-18]. The cook in question is a modified cooking method which utilizes an alkaline cooking liquor just as the sulphate cook, but wherein delignification has been enhanced so that the kappa number of the chemical pulp is lowered without a significant reduction of viscosity. Typically with a Superbatch process, pulp is cooked to a kappa number of 20 or less.
A paper web is formed from the stock of aspen pulp and chemical pulp on a paper machine. Preferably a gap former is used for web forming. In said technique the web is dried between two webs, water being removed in both directions. Thus, as regards printability, an advantageous distribution of the fines is obtained in the direction of the Z axis; the fines are gathered on both surfaces of the base paper web. A xe2x80x9csmilingxe2x80x9d distribution is formed in transversal direction when the fines accompany the leaving water. A paper according to the invention contains substantially much more fibes than for example a traditional spruce groundwood-based LWC. The fines of the aspen and the fillers added to the stock are accumulated on the surfaces of the paper. Because aspen has a rather good brightness and a good brightness stability, it is possible to get abundant amounts of aspen fibers on the surface of the paper. The coating is also accumulated on the surface of such a paper and, thus, a good coverage can be obtained. Therefore, by combining the use of a gap former with the present fiber mixture it is possible to provide a base paper which has rather advantageous printing properties after coating.
As regards the runability of the above-described fiber mixture it is particularly advantageous to set the dosing pH of the stock at 6.8 to 7.2 and the pH of the machine pulp at 7.1 to 7.5, preferably at about 7.1 to 7.3.If necessary a suitable base or acid is used for setting the pH and for adjusting the pH during paper making. The bases used comprise in particular alkali metal bicarbonates or carbonates and alkali metal hydroxides. The acids used include mineral acids and acid salts. The preferred acids are sulphuric acid and its acid salts such as alum, and the preferred base is sodium bicarbonate. The consistency of the headbox is adjusted to 0.6 to 0.8.
By using the invention, the following properties can be obtained for the base paper:
From a base paper of this kind a high-quality fine paper can be produced by coating it twice with a suitable coating colour containing pigments. The coating colour can be applied on the material web in a manner known per se. The method according to the invention for coating paper and/or paperboard can be carried out on-line or off-line by using a conventional coater, i.e. a doctor blade coater, or by film press coating or by surface spraying
According to a particularly preferred embodiment, the paper web is double-coated, whereby the first coating is for example carried out by the film press method, and the second coating is performed by doctor blade coating. The precoating is preferably performed by film press coating e.g. at high speed (at least 1450 m/s, preferably even 1600 m/min or more). Generally, the amount of coating colour applied to the web by the film press method is typically about 5 to 50 g coating colour/m2, whereas the corresponding amount for doctor blade coating is 10 to 60 g coating colour/m2. The coating weights have been calculated from the dry matter of the coating colour.
The solution according to the invention is particularly well suited to coating by using in the coating colour a pigment with a steep distribution whereby the pigment will provide good coverage and the paper will have good opacity. By steep pigment size distribution is meant a distribution in which a maximum of 35% of the particles are smaller than 0.5 xcexcm and preferably a maximum of 15% are smaller Man 0.2 xcexcm.
After coating and supercalendering the fine paper obtained typically has the following properties: